When there is a need for energy storage at voltages greater than a single cell can provide, individual cells can be connected in a series arrangement to generate a higher voltage series connected battery. Mismatches between the cells may cause the cells to experience undervoltage or overvoltage conditions that are undesirable. Some cell chemistries may be particularly sensitive to the undervoltage or overvoltage conditions. The mismatches may be caused by manufacturing tolerances in the initial production of the cells, age difference between cells, or differences in the state of charge. Maintaining the cells in balance in order to avoid undervoltage or overvoltage conditions is a critical design goal that produces a reliable and safe battery having a long operating cycle life.
Maintaining the cells in balance may require charging and discharging individual cells. One method for discharging is resistive loading of individual cells. Though simple and low cost, the energy being discharged by this method is wasted as heat. Another method for charging includes connecting a current source to individual cells. The current source may be derived from the battery and require elaborate multiplexing or switching schemes to enable the current source to be connected to each cell. The elaborate multiplexing/switching scheme may require control circuits external to the battery and the breakdown voltage of the switches must be as high as the voltage of the overall battery stack. The single current source may also limit the charging to a single cell at a time.
An additional method may include transferring charge between adjacent cells using, for example, a capacitor that can be switched therebetween. Such an arrangement may limit the balancing to sets of adjacent cells. Another method may include complicated control and switching arrangements to transfer charge between cells. These systems may also require control circuits external to the battery and the breakdown voltage of the switches to be as high as the voltage of the overall battery stack.
What is needed is a simple means for autonomously transferring charge between cells without the need for elaborate control and switching. Enabling the charge to be transferred from cell to cell enables the components used therein to transfer the charge to have a lower breakdown voltage and thus be smaller and less costly.